1. Field of the Invention
This invention relates to a fastener installation and method for a composite member and, more particularly, to a fastener installation and method in which a shear member is adhesively bonded to the composite member and a pin member extends through the shear member and an aligned opening of the composite member.
2. Background Information
Structures, as for example airframes, are formed by fastening members together. In past years, refinements have been made in the materials used for such fastener joints, and interference fit fasteners, bushings, inserts, tension pre-loads, and adhesives have been used to advance the fastener art.
For example, in U.S. Pat. No. 3,469,490, an adhesive is forced into spaces between the fastener head, shank, and the workpiece layers as the fastener is tightened. The adhesive bonds the fastener to the structure, and precludes non-destructive disassembly of the workpiece.
In U.S. Pat. No. 3,835,615, a sleeve is bonded in place in a fastener hole. Forces are transmitted through bearing contact between a fastener shank, the sleeve, and the workpiece. The fastener may be removed, but the sleeve remains bonded in place and thus precludes non-destructive disassembly of the workpiece.
In U.S. Pat. No. 4,010,519, metal plates are bonded to surfaces of a composite member. A shank extending through the member is also bonded to the member, and has a central hole for receiving a fastener. Forces are transmitted in bearing between fastener, shank, and fibers of the composite member.
In British Patent 1 455 235, an insert has a shank cemented in the fastener hole of a fibrous material, and a head portion cemented in a recess in a surface of the member. The insert is said to overcome fastener hole deformation caused by vibration stress on inserts. The insert is also said to transmit two-thirds of the shear forces through the shank portion and one-third through the head portion. Substantial shear forces remain transmitted in bearing to the fibrous material.
Combinations of adhesive bonds and fasteners have produced stronger and more reliable fastener joints in metal structures. Faying surfaces of such structures, i.e., opposing surfaces of adjacent members, are bonded together for shear resistance and fasteners such as bolts or rivets extend through the members to secure the members together against tension or peeling forces.
In recent years, the need for high strength, light weight airframes has led to an increased use of structural members formed of fiber-reinforced composite materials. The use of fasteners and faying surface adhesive bonds has been readily adapted to the permanent joints of such structures. In such joints, the limited strength of the composite member in bearing is replaced with the very stiff and strong shear strength of the faying surface adhesive bond.
However, many fastener joints must allow disassembly and reassembly of the structure so that a member or piece of equipment within the structure can be removed, inspected, repaired, or replaced. Faying surface adhesive bond techniques have not heretofore been satisfactorily applied to these kinds of fastener joints because of the need to periodically open and close the fastener joint.
A further problem in securing composite members is that, in typical flush mounted single shear overlap joints, a countersunk pin member extends through the overlapping members and engages the fibers of the composite members in bearing contact to transfer shear loads. Additionally, moment forces from the offset shear loads applied by the composite members bend the pin member, and the head of the pin member gouges into the composite member and buckles and delaminates the plies of the composite member, reducing the strength of the fastener joint. Fastener hole deformation also occurs in such joints from loads applied to bushings or inserts loosened by vibration and other stresses. Eventually, the fastener joint becomes irreparable, and the damaged composite member must be replaced at significant cost and with substantial "down time."
Moreover, the yielding of composite material in tension or compression is negligible. While metallic members can develop full static strength in regions between fastener holes because the ductility of the metal produces a benign stress concentration profile, the high stress concentrations at fastener holes in composite members result in significant static strength reductions for such members as compared to metallic members. Lastly, if the number of fasteners used in composite structures can be minimized, costs can be reduced.
Consequently, the stronger a composite fastener installation can be made, the more likely it is that a composite structure will withstand unanticipated shear loads without damage or failure.
It is therefore an objective of this invention to provide a high-strength fastener installation for composite members which eliminates bearing loads upon the fibers of the composite members, which enables disassembly and reassembly of the structure for inspection, repair, removal, or replacement of members or internal equipment, which increases the static strength of fastener joints in composite members, and which reduces the costs of composite structures. It is another objective of the invention to provide a method for securing composite members to other members which overcomes the difficulties and problems discussed above.